1. Field of the Invention
This invention relates to receivers and more specifically to methods and apparatus for determining data rates of signals received by a receiver.
2. Description of the Related Art
One of the primary uses of cellular communications system is for the transmission of voice. Voice is sampled and then compressed into a digital data stream for transmission and then transmitted to/from a base station and a mobile user. One of the key features of common voice compression algorithms is that it takes more bits to represent speech when someone is talking as compared to when someone is not talking (listening to the other person speaking). There are also different sounds in speech, which go require varying amounts of bits to represent. Therefore, the data rate that should preferably be transmitted to/from a base station and a mobile user is variable. Furthermore, the data rate depends on the person speaking at the transmitter and the receiver has no way of predicting what the data rate will be at any given point in time. Therefore, the receiver must have a method of determining what the received data rate is based on the received data.
Some conventional methods of determining what the received data rate is based on the received data involve the determination of whether various measures exceed a certain threshold. Such measures include the outcome of a cyclical redundancy check (CRC) and the values of Viterbi metrics, Yamamoto Quality Metrics, and Symbol Error Rates. For example, in a system with 4 possible rates, the CRC may be checked for the two highest rates, and if the CRC passes for these two rates, the system determines that one of these two rates is the received rate. To select between these two rates, one or more of the other measures may be compared against thresholds and the rate is selected based upon the outcome of this comparison.
It would be desirable to improve upon the accuracy of such conventional methods.
The present invention comprises methods and apparatus for determining the rate at which data was encoded when such data is received at a receiver. According to the present invention, the rate is determined by computing, for a plurality of possible rates, a final test statistic based on a plurality of measures. The final test statistics are compared and based upon certain selection criteria (for example, without limiting the foregoing, which final test statistic corresponds the highest value), the rate is selected. In the preferred embodiment, the measures comprise statistics based on the cyclical redundancy check, Viterbi metrics, re-encoded symbol error rate, and distance to next largest Viterbi metric.